1. Field of the Invention
The present invention relates to a glass substrate for magnetic information recording medium. More particularly, the present invention provides a glass substrate having the properties required of future information recording medium substrate, which glass substrate has low specific gravity, high Young's modulus and excellent Vickers hardness in response to the future densification of magnetic information recording media, as well as has a highly smooth surface roughness after processing and excellent head sliding properties.
The term “information recording medium” according to the present invention means a magnetic information recording medium that can be used in fixed type hard disks, removable hard disks or card type hard disks, which are used as the hard disks for personal computers; hard disks for digital video cameras, digital cameras or audio systems; hard disks for automobile navigators; hard disks for mobile telephones; or hard disks for various electronic devices.
2. Description of the Related Art
In order to cope with recent development of personal computers for multi-media purposes and to cope with handling of large amounts of data such as video files and audio files by digital video cameras, digital cameras and portable audio devices, there is a demand for magnetic information recording devices with larger capacities. As a result, it is increasingly required of magnetic information recording media to acquire higher recording density year after year.
In response to this, adoption and mass production of perpendicular magnetic recording system are in progress. This perpendicular magnetic recording system is required to have more excellent heat resistance of substrates and smoothness of surfaces than conventional systems. Furthermore, conventionally, most of magnetic information recording apparatuses are used in a static environment, such as in the use in personal computers; however, in recent years, more of the recording apparatuses are put to use in a dynamic environment, which is represented by portable audio devices and the like. For this reason, it is becoming increasingly important that substrates for information recording media of next generation have lower specific gravity so as to reduce the burden on spindle motors, and higher mechanical strength for preventing hard disk crash. However, having high mechanical strength and excellent mass producibility is in a contradictory relationship with having low specific gravity, and thus research is being conducted on how these factors can be put in balance in the substrates for information recording media of next generation, for which the use in a dynamic environment is taken into consideration.
Materials that are used in the substrates for information recording media include aluminum (Al) alloys, glass, crystallized glass and the like, and glass and crystallized glass have an advantage over Al alloys in view of having high Vickers hardness and the like. Furthermore, crystallized glass generally has an advantage over glass in view of having higher Young's modulus. However, in recent years, there is a demand for a higher level of post-processing surface smoothness, but it is becoming more difficult to obtain the required surface smoothness in the crystallized glass. On the other hand, conventional glass substrates are not sufficiently satisfactory in terms of the Young's modulus that is required of the substrates for information recording media of next generation, and even if those currently established processing technologies are applied, desired surface properties may not be obtained.
In the case of using glass materials, direct press method is utilized by which molten glass is directly pressed, in order to produce disk-shaped substrates having a thickness of 1 mm or less at low cost.
In the direct press method, arsenic or antimony components have been used as refining agents so as to remove bubbles from molten glass when glass is melted. In recent year, however, there are rising concerns about the adverse effects of the refining agents on human body and environment, and thus it is requested to reduce the content, or not to use the refining agents at all.
Japanese Patent Application Laid-Open (JP-A) No. 2005-302289 discloses a substrate for an information recording medium made of glass, but the substrate does not have sufficient Vickers hardness and does not have sufficiently satisfactory properties that are required for the use as a substrate for an information recording medium of the next generation. The substrate also tends to have a low ratio of Young's modulus to the specific gravity.
JP-A No. 2001-19467 proposes a crystallized glass for magnetic disks, but this crystallized glass exhibits a difference in processing or a difference in the etching rate between the precipitated crystals and the glass blank, so that the crystallized glass cannot sufficiently satisfy the surface properties of the currently required level of Ra<2 A (angstrom).